A III-V gallium nitride (GaN)-based compound semiconductor such as GaN, GaInN, and AlGaInN has a wide bandgap of 2.8 to 6.8 eV, and attention is focused on such a GaN-based compound semiconductor as the material of a semiconductor light emitting device capable of emitting light in a red to UV region.
As a GaN-based semiconductor light emitting device using a III-V GaN-based compound semiconductor as a component, a blue or green light emitting diode (LED) and a GaN-based semiconductor laser device oscillating in a violet region of about 405 nm, for example, have been developed and put into practical use.
One problem in fabrication of a GaN-based semiconductor light emitting device is that it is difficult to find a substrate lattice-matched to GaN-based compound semiconductor layers. At present, a sapphire substrate is frequently used as the substrate for the GaN-based semiconductor light emitting device.
The reasons for such frequent use of the sapphire substrate are that the sapphire substrate has chemical stability required in crystal growth of the GaN-based compound semiconductor layers at a growing temperature of about 1000° C., that the crystal quality is good, and that a substrate having a relatively large diameter can be supplied economically and stably.
The lattice constant of the sapphire substrate is different 10% or more from the lattice constant of GaN. Therefore, in forming the GaN-based semiconductor light emitting device on the sapphire substrate, a buffer layer such as a GaN layer is generally grown on the sapphire substrate at a low temperature, and a GaN-based compound semiconductor single-crystal is grown on this low-temperature buffer layer, thereby relaxing the difference in lattice constant.
However, even by providing the low-temperature buffer layer and growing the GaN-based compound semiconductor layers on the low-temperature buffer layer, the density of crystal defects becomes high. Accordingly, it is difficult to grow high-quality GaN-based compound semiconductor layers, so that it is difficult to fabricate the GaN-based semiconductor light emitting device with high reliability.
To solve this problem, GaN-ELO (Epitaxially Laterally Overgrowth) is performed in addition to the interposition of the low-temperature buffer layer. Thus, a GaN-ELO structure layer is formed and the GaN-based compound semiconductor layers are grown on this GaN-ELO structure layer.
Referring to FIG. 5, there is shown a sectional structure of a GaN-based semiconductor laser device having such a GaN-ELO structure layer formed on a sapphire substrate. FIG. 5 is a sectional view showing the configuration of the GaN-based semiconductor laser device using the sapphire substrate.
As shown in FIG. 5, the GaN-based semiconductor laser device 10 includes the sapphire substrate 12, the GaN-ELO structure layer 14 formed on the sapphire substrate 12 by ELO, and a multilayer structure composed of an n-type GaN contact layer 16, n-type AlGaN clad layer 18, n-type GaN guide layer 20, GaInN active layer 22 having a multiple quantum well (MQW) structure, p-type GaN guide layer 24, p-type AlGaN clad layer 26, and p-type GaN contact layer 28 sequentially grown on the GaN-ELO structure layer 14 by MOCVD.
An upper portion of the p-type AlGaN clad layer 26 and the p-type GaN contact layer 28 are formed as a stripe-shaped ridge 30 located between a seed crystal portion and a junction portion of the GaN-ELO structure layer 14.
Further, the remaining portion of the p-type AlGaN clad layer 26, the p-type GaN guide layer 24, the active layer 22, the n-type GaN guide layer 20, the n-type AlGaN clad layer 18, and an upper portion of the n-type GaN contact layer 16 are formed as a mesa 32 parallel to the ridge 30.
An SiO2 film 34 having an opening at a position over the p-type GaN contact layer 28 is deposited on the opposite side surfaces of the ridge 30 and on the remaining portion of the p-type AlGaN clad layer 26.
A p-side electrode 36 of a Pd/Pt multilayer metal film is formed on the p-type GaN contact layer 28. A pad metal 37 is provided as a leading electrode on the SiO2 film 34 so as to be electrically connected through the opening of the SiO2 film 34 to the p-side electrode 36. With this structure, a low-resistance Schottky p-side electrode can be formed. The pad metal 37 is formed from a Ti/Pt/Au multilayer metal film.
Further, an n-side electrode 38 of a Ti/Pt/Au multilayer metal film is provided on the n-type GaN contact layer 16 so as to be exposed to another opening of the SiO2 film 34.
The lattice mismatch between the sapphire substrate and the GaN-based compound semiconductor layers is relaxed by adopting the ELO process as mentioned above. However, any other problems associated with the lattice mismatch remain as far as the sapphire substrate is used as the substrate of the GaN-based semiconductor light emitting device. Further, since the sapphire substrate is dielectric, there is a limit to the arrangement of the electrodes.
Accordingly, it is strongly desired to realize a GaN substrate. However, it is conventionally very difficult to industrially fabricate a GaN substrate having a large diameter with reduced crystal defects which can be used as the substrate of the GaN-based semiconductor light emitting device.
Such a large-diameter GaN substrate based on a novel technique has recently been on the way to realization.
The configuration of a novel GaN substrate will now be described with reference to FIGS. 6A and 6B. FIGS. 6A and 6B are a perspective view and a sectional view of a GaN substrate, respectively, for illustrating core portions and a low-density defect region.
As shown in FIG. 6, the GaN substrate 40 has a low-density defect region 42 and a high-density defect region (which will be hereinafter referred to also as core portions) 44 having a crystal defect density higher than that of the surrounding low-density defect region 42. The core portions 44 are periodically arranged on the substrate surface and passed through the thickness of the substrate.
The arrangement pattern of the core portions 44 is not fixed, but it may include dotted dispersive patterns such as a hexagonal lattice pattern shown in FIG. 7A, a square lattice pattern shown in FIG. 7B, and a rectangular lattice pattern shown in FIG. 7C.
The arrangement pattern of the core portions is not limited to the above-mentioned intermittent or dispersive patterns, but may include an intermittent linear pattern of dotted core portions 44 as shown in FIG. 8A and a continuous linear pattern of belt-shaped core portions 44 as shown in FIG. 8B.
The above-mentioned GaN substrate has been developed by improving the technique disclosed in Japanese Patent Laid-open No. 2001-102307 and controlling the positions of the core portions generated in the low-density defect region.
The basic mechanism of crystal growth of a GaN single-crystal will now be described. The GaN single-crystal is grown with an inclined facet being maintained, thereby propagating dislocations and gathering them to a predetermined position. The region grown by this facet becomes a low-density defect region owing to the movement of dislocations.
On the other hand, a high-density defect region (core portions) having a clear boundary is generated and grown under the inclined facet, and the dislocations are gathered at the boundary of the high-density defect region or inside thereof, then disappearing or being accumulated.
The shape of the facet differs according to the shape of the high-density defect region. In the case that the high-density defect region has a dotted pattern, each dot forms a bottom surrounded by the facet to form a pit by the facet.
In the case that the high-density defect region has a striped pattern, each stripe forms a valley having inclined facets on the opposite sides. These facets are opposite surfaces of a triangular prism in its laid condition.
Further, the high-density defect region may have some states. For example, the high-density defect region may be polycrystalline. Further, the high-density defect region may be a single crystal and minutely inclined with respect to the surrounding low-density defect region. Further, there is a case that the C-axis of the single crystal is inverted with respect to the surrounding low-density defect region. The high-density defect region has a clear boundary and it can be distinguished from the surrounding low-density defect region.
The crystal growth of the GaN single-crystal can be advanced with the surrounding facet being not buried but maintained.
Thereafter, the surface of the GaN grown layer is ground and polished to become a flat surface which can be used as a substrate surface.
The high-density defect region (core portions) may be formed by preliminarily forming a seed at a position where each core portion is to be formed in the crystal growth of GaN on a base substrate.
The seed may be formed by forming an amorphous or polycrystalline layer in a minute region. By epitaxially growing GaN on this amorphous or polycrystalline layer, the high-density defect region or core portion can be formed in the above seed region.
A specific manufacturing method for the GaN substrate will now be described. First, a base substrate on which a GaN layer is to be grown is prepared. The composition of the base substrate is not limited. For example, a general sapphire substrate may be used. However, in consideration of removal of the base substrate in a subsequent step, a GaAs substrate or the like is preferable.
Thereafter, seeds of SiO2 layers, for example, are formed regularly, e.g., periodically on the base substrate. The pattern of the seeds is a dotted pattern or a striped pattern according to the shape and pattern of the core portions.
Thereafter, a thick film of GaN is grown by HVPE (Hydride Vapor Phase Epitaxy). After growth of the thick film, facets are formed on the surface according to the pattern of the seeds. For example, in the case that the pattern of the seeds is a dotted pattern, pits by the facets are regularly formed. In the case that the pattern of the seeds is a striped pattern, prismatic facets are formed.
After growing the thick GaN layer, the base substrate is removed and the thick GaN layer is next ground and polished to obtain a flat substrate surface, thus fabricating the GaN substrate. The thickness of the GaN substrate can be freely set.
In such a GaN substrate fabricated above, the c-plane is a principal plane, and dotted or striped core portions each having a predetermined size are regularly formed. The GaN single-crystal region except the core portions is a low-density defect region having a dislocation density remarkably lower than that in the core portions.
The GaN substrate prepared by the above method has good crystallinity similar to that of the GaN layer grown by applying the ELO process. Further, the width of the low-density defect region in the GaN substrate is ten times or more the width of the low-density defect region in the GaN layer grown by the ELO process, and the width of the high-density defect region (each core portion) in the GaN substrate is narrower (e.g., tens of micrometers) than that in the GaN layer grown by the ELO process.
For example, a (0001) n-type GaN substrate has been developed such that core portions are spaced by a distance of 400 μm so as to extend in a [1-100] direction, and that a low-density defect region is present between the adjacent core portions. The dislocation density in the (0001) n-type GaN substrate developed above in relation to the distance (μm) from the center of one of the adjacent core portions is shown in FIG. 9. As apparent from FIG. 9, the region having a dislocation density of less than 1.0×106 cm−2 is present in the range of more than 150 μm, and the minimum dislocation density in this region is 2.8×105 cm−2. In FIG. 9, the value 0 on the horizontal axis means the center of one of the adjacent core portions, and the value 400 on the horizontal axis means the center of the other core portion.
Thus, the GaN substrate is a substrate having excellent crystallinity as mentioned above. Accordingly, attempts are actively being made to fabricate a GaN-based semiconductor laser device using the GaN substrate, e.g., a GaN-based semiconductor laser device having the same multilayer structure as that of the above-mentioned GaN-based semiconductor laser device.
However, in operating the GaN-based semiconductor laser device using the GaN substrate, there arises a problem such that a current injected from the p-side electrode through the pad metal leaks without contributing to light emission, that is, the injection current flows so as to be short-circuited from the p-side electrode to the n-side electrode or to the ground.
As a result, a current-to-light conversion efficiency becomes low and there is also a case that no light is emitted.
The above problem is described by illustrating a GaN-based semiconductor light emitting device such as a semiconductor laser device. This problem, however, occurs also in a general GaN-based semiconductor device including not only a GaN-based semiconductor light emitting device, but an electron transit device, for example.
It is therefore an object of the present invention to provide a GaN-based semiconductor device using a GaN substrate which can reduce the current leak.